High performance hyperspectral imagers are valuable instruments in providing calibrated radiometric imagery and determining the spectral composition of detected electromagnetic radiation. Hyperspectral sensors behave like a radiometric sensor by measuring the calibrated light flux which is received at each pixel of the detector emanating from an object through reflection or emission. Equally as important, these sensors also differentiate the spectral composition of the electromagnetic radiation. The resultant information combines a conventional two-dimensional radiometric image with a third dimension containing spectrophotometric response across a broad spectral range, some of these responses are unique indicators of materials and processes. The method is entirely non-contact and non-invasive, making it a common imaging method in the fields of astronomy, agriculture, biomedical imaging, geosciences, physics, and surveillance, for example.
Various hyperspectral imaging applications use an on-board calibrator that includes one or more light sources that provide reference photon fluxes that span the spectral wavelength range of interest, for example, from visible (e.g., ˜400 nm-700 nm) to near-infrared (NIR; e.g., ˜800 nm-1 μm) to short-wave infrared (SWIR; e.g., ˜1 μm-2 μm) to mid-wave infrared (MWIR; e.g., ˜3 μm-5 μm) to long-wave infrared (LWIR; e.g., ˜8 μm-15 μm). One example of a typical calibrator light source is a tungsten-halogen lamp. When tungsten-halogen lamps are operated at various temperatures, they provide sufficient photon flux across the above-mentioned wavelength range with the exception of the blue range. Higher temperature operations are presently limited by the melting temperature of the filament and thus limit the blue output. Further, multiple power cycling and corresponding temperature cycling events induce stress into the brittle tungsten filament, leading to fracture, and ultimate failure. High temperature light sources, such as Xenon lamps, exhibit a broadband and high output emission. However, these lamps require a large amount of power and generate a large amount of heat, requiring an additional cooling subsystem. Additionally, the high temperature plasma generated imparts stresses to the glass envelope, leading to premature failure after limited thermal cycling (e.g., ˜50 cycles).
Recently, commercial lighting technology has advanced to low-power light emitting diodes (LEDs) that are commonly used in traffic lights, automobile lights, and home track lighting, for example. These applications have used specially doped semiconductors that emit in narrow bands in the blue, green, or red wavelength ranges, ±0.02 μm approximately. Further, various lighting applications use a blue LED in combination with a phosphor to down-convert to a continuum of wavelengths from green to red. Although such advances have resulted in bright white sources to the human eye, the blue emission (e.g., from ˜0.4 μm-0.43 μm) is still lacking.
Thus, there remains a continuing need for hyperspectral calibrators that are compact and lower power, emit over the full visible to LWIR spectral range, including the blue emission, and exhibit temperature cycling stability.